SPEAKER_1: Our universe is filled with secrets and mysteries, leaving us with many questions to be answered.
SPEAKER_1: Now, more than ever, we find ourselves searching for those answers as the very fabric of space, science, and society are converging.
SPEAKER_1: This is StarTalk.
SPEAKER_3: Now, here's your hosts, astrophysicist, Dr.
SPEAKER_3: Neil deGrasse Tyson, and comedian, Lynn Coplitz, StarTalk.
SPEAKER_4: Welcome back to StarTalk.
SPEAKER_4: I'm your host, astrophysicist, Neil deGrasse Tyson, and my co-host, Lynn Coplitz, professional comedian, is actually on the road right now.
SPEAKER_4: She's performing at the Pittsburgh Improv.
SPEAKER_4: You might catch her tonight, if you happen to be in the area.
SPEAKER_4: And she's also an actress and appears in Z-Rock.
SPEAKER_4: On the Independent Film Channel on Sunday nights.
SPEAKER_4: Check it out.
SPEAKER_4: You can go to Lynn Coplitz's homepage at lynncoplitz.com.
SPEAKER_4: That's Lynn with an E.
SPEAKER_4: Again, I'm your host, Neil deGrasse Tyson.
SPEAKER_4: Welcome back to StarTalk.
SPEAKER_4: In lieu of my charming co-host, Lynn Coplitz, I snatched from the halls of Princeton University an old-time friend and colleague of mine, Professor J.
SPEAKER_4: Richard Gott.
SPEAKER_4: Richard Gott, welcome to StarTalk Radio.
SPEAKER_2: Hi, Neil.
SPEAKER_4: Richard Gott is one of the world's experts on all kinds of exotic astrophysics.
SPEAKER_4: And today's subject is going to be time travel.
SPEAKER_4: Is time travel possible?
SPEAKER_4: And what sorts of frontier astrophysical phenomena, black holes, warp space, wormholes, what sorts of astrophysical phenomena enable it or prevent it?
SPEAKER_4: That's today's show on StarTalk.
SPEAKER_4: Rich, every time when we begin StarTalk, we find out what our good friend Bill Nye has to say about black holes and Big Bang and the universe.
SPEAKER_4: You're going to hang for the whole hour.
SPEAKER_2: Sure.
SPEAKER_4: And let's see what Bill Nye says to start us off.
SPEAKER_4: Check him out.
SPEAKER_5: Hey, hey, Bill Nye the Science Guy here.
SPEAKER_5: As the singer Hank Williams pointed out, we'll never get out of this world alive.
SPEAKER_5: For us, the conveyor belt of time goes one way and we're on it.
SPEAKER_5: No way to live forever.
SPEAKER_5: And getting out of here, forget it.
SPEAKER_5: Gravity's got us stuck.
SPEAKER_5: But since this is StarTalk Radio, we can tell you there is a way out.
SPEAKER_5: Find a black hole and fall in.
SPEAKER_5: That's a star, or was a star so massive that nothing can get out, not even pure energy.
SPEAKER_5: Light is sucked in along with everything else.
SPEAKER_5: In there, you could end up in another part of the universe or another universe at another time.
SPEAKER_5: You're still dead, though.
SPEAKER_5: This black hole business is not intuitive.
SPEAKER_5: And we may have just discovered an intermediate version and special clouds of interstellar gas called blobs detectable with X-rays.
SPEAKER_5: Fundamentally, from our universe, is there a way to see to some other side?
SPEAKER_5: There has to be a lot more to the story of stars than we know right now.
SPEAKER_5: But StarTalk listeners, you and I, can't help but struggle and strive to understand the world that keeps us alive.
SPEAKER_5: Listen on.
SPEAKER_4: That was Bill Nye, frenetic as he is in every minute that he gives us.
SPEAKER_4: You're listening to StarTalk Radio.
SPEAKER_4: You can call us at 1-877-5-STARTALK if you have a question on the frontier of astrophysics, be it about Big Bang, the black hole that we've been discovering, wormholes, and in particular, today's subject, time travel.
SPEAKER_4: So Rich, my guest host for today, Rich, is time travel possible?
SPEAKER_2: Well, yes.
SPEAKER_2: Einstein showed that time travel to the future was possible in his 1905 theory of special relativity.
SPEAKER_4: And you say that as though you've done it.
SPEAKER_4: Yeah, it's possible.
SPEAKER_4: Like, yeah, it's possible to go to Baltimore, you know.
SPEAKER_2: Well, we do have time travelers with us even today.
SPEAKER_2: The greatest time traveler up to date is Sergei Krikulov.
SPEAKER_2: He's a Russian astronaut.
SPEAKER_2: He spent time up on the Mir space station and in a few shuttle flights.
SPEAKER_4: Now, the Mir space station preceded the current International Space Station.
SPEAKER_2: That's right.
SPEAKER_4: In case people forgot that, just to clue them in, yeah.
SPEAKER_2: And so the mechanism that he's using there is that Einstein showed that in special relativity that moving clocks tick slowly.
SPEAKER_4: Well, how long was he on Mir?
SPEAKER_2: Well, the total time he spent in orbit was about 803 days.
SPEAKER_4: So years, he spent years.
SPEAKER_2: Yeah, yeah.
SPEAKER_4: Yeah, and so the space station, the Mir, in orbit is going like 18,000 miles an hour.
SPEAKER_4: So is that fast enough to have measurable effects?
SPEAKER_2: It slows down your clock just a little.
SPEAKER_2: So in fact, he came back after all of his voyages.
SPEAKER_2: He was 1 48th of a second younger than he would have been if he'd stayed home.
SPEAKER_4: So he traveled to the future 1 48th of a second.
SPEAKER_2: 1 48th of a second.
SPEAKER_2: So when he got back, see, he found the Earth to be 1 48th of a second toward the future.
SPEAKER_2: Now, that doesn't seem like much.
SPEAKER_4: No, not only does it not seem like much, it isn't much, Rich.
SPEAKER_2: Well, someone asked me once, well, why is it so easy to travel in space but so hard to travel in time?
SPEAKER_2: And the answer is we haven't been very far in space either.
SPEAKER_2: The furthest we've been in space is to the Moon, which Einstein would tell us is 1.3 light seconds away.
SPEAKER_4: You can just calculate that.
SPEAKER_4: You wouldn't need Einstein to tell you it's 1.3.
SPEAKER_2: Well, he would tell you that if you're going to compare distances in space with distances in time, you should use the speed of light.
SPEAKER_2: So we say Alpha Centauri is four light years away.
SPEAKER_4: The nearest star to the Sun, star system.
SPEAKER_2: So it takes light four years to get here.
SPEAKER_2: So we've been 1.3 seconds in light seconds in space and 1 48th of a second in time.
SPEAKER_2: So Mr.
SPEAKER_2: Krikulov is like Lindbergh.
SPEAKER_2: He's like traveled a distance in time equal to going across the Atlantic, basically.
SPEAKER_4: Oh, I see.
SPEAKER_4: Okay.
SPEAKER_4: So that's just the beginning then, just as Lindbergh...
SPEAKER_2: That's the first step.
SPEAKER_4: By the way, Rich, you have a book called Time Travel in Einstein's Universe.
SPEAKER_4: It came out a couple of years ago, but it's available in paperback, published by Houghton Mifflin.
SPEAKER_4: And we'd like to actually give away some free copies of that book to callers whose questions may have earned the right to receive a free book.
SPEAKER_4: We'll be listening for your calls later in the show.
SPEAKER_4: So we got Krikulov, Krikulov?
SPEAKER_2: Krikulov.
SPEAKER_4: Krikulov.
SPEAKER_2: Sergei Krikulov.
SPEAKER_2: Now the previous guy who had the record was Sergei Avdeyev.
SPEAKER_2: So he was the 50th of a second.
SPEAKER_2: So apparently it's good to be named Sergei.
SPEAKER_4: So the Russians, so we don't have any Americans who are in the running for this, I guess.
SPEAKER_2: Not in the running currently.
SPEAKER_4: So we're nicer to our people by not leaving them in orbit for years and forgetting about them.
SPEAKER_2: I think that's right.
SPEAKER_4: So do you have like a sort of layman's account for how you can actually travel into the future using Einstein's rules?
SPEAKER_4: What is it about the physics of relativity that enables this?
SPEAKER_2: Well, we all know that Einstein was a genius.
SPEAKER_2: So what did he do that really made him a genius?
SPEAKER_2: Well, special relativity he built off of two postulates.
SPEAKER_2: One was that motion is relative.
SPEAKER_2: And you've seen this when you're on an airplane going at 500 miles an hour and the windows are down, you're watching some bad movie.
SPEAKER_2: And it seems just like you're on the ground.
SPEAKER_4: Yeah, if there's no turbulence.
SPEAKER_2: If there's no turbulence and you're not turning.
SPEAKER_2: In fact, sometimes you actually are on the ground.
SPEAKER_4: Most times lately.
SPEAKER_2: Well, that was it, that motion is relative.
SPEAKER_2: You can't tell whether you're moving or not.
SPEAKER_2: He thought that that applied to Newton's laws of physics and he thought that should apply to all the laws of physics, including electrodynamics.
SPEAKER_4: electrodynamics is a study of properties of light, basically.
SPEAKER_2: Yes.
SPEAKER_4: Light and particles interacting with them, yeah?
SPEAKER_2: Yes, and so the second postulate was that the speed of light should always be observed by you.
SPEAKER_2: A light beam should pass you at always the same speed, 300,000 kilometers a second.
SPEAKER_2: And that was because Maxwell showed that the speed of light could be calculated from a ratio of magnetic to electric forces.
SPEAKER_2: And when he got that velocity, he said, Eureka, this is the velocity astronomers have already measured to be the velocity of light.
SPEAKER_2: So I know that electromagnetic waves, which I've just calculated, is light.
SPEAKER_2: So Einstein thought that we should see that always at that speed.
SPEAKER_2: If we didn't, we would know we were moving.
SPEAKER_2: If we saw it going by 200,000 kilometers a second, we'd say, hey, I must be moving at 100,000.
SPEAKER_2: And that wasn't allowed by the first postulate.
SPEAKER_2: So with those two postulates, he proved a lot of theorems that you could then check.
SPEAKER_2: He did thought experiments.
SPEAKER_2: So one of the experiments that he thought of was...
SPEAKER_4: So as a theorist without a budget to create a lab, he thinks up experiments in his head.
SPEAKER_2: He thought up the experiment.
SPEAKER_2: Nobody really done anything like this before.
SPEAKER_2: He thought up an experiment.
SPEAKER_2: And one of the experiments was, he said, suppose I have a light clock, and I just have a mirror at the bottom and a mirror at the top, I'll bounce the light beam back and forth.
SPEAKER_4: This is a mirror on the floor and a mirror on the ceiling.
SPEAKER_2: That's right.
SPEAKER_4: Start a beam of light bouncing between the two, and there it goes.
SPEAKER_2: And it'll take a number of nanoseconds to go from the top to the bottom.
SPEAKER_4: Billionths of a second.
SPEAKER_2: Yes, if it's three feet tall, your clock will take three nanoseconds to beat.
SPEAKER_4: A foot per nanosecond.
SPEAKER_2: That's the velocity of light.
SPEAKER_2: It's very easy to remember in those.
SPEAKER_4: So you're sitting now for me three billionths of a second away from me.
SPEAKER_2: In fact, you're seeing me not as I am now, but as I was three billionths of a second ago.
SPEAKER_4: And you look marvelous.
SPEAKER_2: In the past.
SPEAKER_2: I'm a bit younger than I really am.
SPEAKER_2: I look a bit younger.
SPEAKER_2: The further you move away, the younger you look.
SPEAKER_2: So this is something to remember.
SPEAKER_2: Anyway, you have the light clock.
SPEAKER_4: So I'm in this closed room, and I see the light beam just going back and forth, and I can keep time this way.
SPEAKER_2: That's a good clock.
SPEAKER_2: Now suppose there's an astronaut moving by you at four-fifths the speed of light, zipping by, from left to right.
SPEAKER_2: Now he's got a light clock, too, just like yours.
SPEAKER_2: And he sees it going up and down in the light clock from the floor to the ceiling.
SPEAKER_4: In his room.
SPEAKER_2: In his room.
SPEAKER_2: But his room's moving relative to yours.
SPEAKER_2: So when you look at his light beam, you see it go from the bottom, down here over on the left, to the upper right as he's moving from left to right.
SPEAKER_2: By the time he gets to the top, he's moved over to the right.
SPEAKER_2: So that light beam has gone on a longer diagonal path, as far as you're measuring it.
SPEAKER_4: It's gone on the hypotenuse of that triangle.
SPEAKER_2: The hypotenuse of the triangle.
SPEAKER_4: We all remember the hypotenuse.
SPEAKER_2: We all remember that.
SPEAKER_2: And it's longer than one of the sides.
SPEAKER_4: By the way, you're listening to Professor Rich Gott from Princeton University, a colleague of mine from the Department of Astrophysics there, talking cosmic stuff.
SPEAKER_4: Go on.
SPEAKER_4: He's explaining how you can move into the future using relativity.
SPEAKER_4: So go on.
SPEAKER_2: So when I look at his light clock, his light clock has got to tick slow relative to mine.
SPEAKER_2: If he's moving at four-fifths the speed of light, that diagonal path is five feet that I measure.
SPEAKER_2: So I must say it takes five nanoseconds for his clock to tick.
SPEAKER_2: So I must see his clock ticking at three-fifths the rate of my clock.
SPEAKER_2: Now here's the interesting part.
SPEAKER_2: All of his clocks have to tick slower by that same factor.
SPEAKER_4: Not just his light clock.
SPEAKER_2: Not just his light clock.
SPEAKER_2: His heart has to beat slower.
SPEAKER_2: Otherwise, he'd notice that relative to his heart, which is also a clock, that his light clock is ticking slow.
SPEAKER_2: And so because he can't know whether he's moving or not, he has to age more slowly too.
SPEAKER_4: So you're saying Einstein's two postulates require that he's aging less if his two postulates are correct.
SPEAKER_2: That's right.
SPEAKER_4: And lo and behold, experiments show that in fact this is the case.
SPEAKER_2: They do.
SPEAKER_2: They took atomic clocks on airplanes, flew them east around the world where the velocity of the plane would add to the rotational velocity of the Earth, and they came back slow by about 50 nanoseconds, which is what Einstein would have predicted.
SPEAKER_4: And these clocks are accurate enough to measure that.
SPEAKER_4: When clearly you're a Russian astronaut, you could not have said, or let's check your heartbeat, see if it's 1 48th of a second too slow.
SPEAKER_4: You couldn't have done that.
SPEAKER_4: So it's really the calculation that tells you that he went into the future by that much.
SPEAKER_2: But the implications of this are large because if you get on a spaceship and you go at 99.995% the speed of light out to a star 500 light years away and then you turn around and you come back.
SPEAKER_2: When you get back, the Earth will be 1,000 years older, but you will have only aged 10 years.
SPEAKER_4: This is how you move into the future.
SPEAKER_4: Now who would want to do that because everyone would have just forgotten about you?
SPEAKER_2: Well, I think that would be really interesting.
SPEAKER_2: So you would do this.
SPEAKER_2: Oh, sure.
SPEAKER_4: We love you here in the present, Rich.
SPEAKER_2: But the year of 3000 would be really interesting.
SPEAKER_4: You would home if Earth is still here, if humans are still here.
SPEAKER_2: Look, it took Marco Polo 24 years to go visit China and come back to Europe and tell people about it.
SPEAKER_2: And so if you went on an accelerating rocket ship, you could accelerate it, 1G acceleration, perfectly comfortable.
SPEAKER_2: It would take you about 24 years going out and back as far as your clock was concerned.
SPEAKER_2: You would arrive back there and it would be a thousand years in the future.
SPEAKER_4: We are going to get back to this because we know that there have been movies we have all seen that involve time travel.
SPEAKER_4: And I want to get from you your expert view on whether those movies got it right or wrong or what was fun or what irked you when you saw them.
SPEAKER_4: We are going to take a break from StarTalk Radio.
SPEAKER_4: I am Neil deGrasse Tyson, your host.
SPEAKER_4: And call us and tell us what your favorite time travel movie has been or whether you want to travel into the past or the future if you had that choice.
SPEAKER_4: We are going to learn how to travel into the past as well and learn all about what kinds of paradoxes that might create as we have learned from films as well.
SPEAKER_4: We will be back after a break.
SPEAKER_4: 1-877-5-STAR-TALK is our number.
SPEAKER_1: Whether you're a space cadet or a rocket scientist, we want to hear from you.
SPEAKER_1: The phone lines are open.
SPEAKER_1: Call now.
SPEAKER_1: This is StarTalk.
SPEAKER_4: That would be 1-877-5-STAR-TALK.
SPEAKER_4: If you have a question about time travel, wondering whether it's possible, you have a question from my host, my co-host for today's episode, and he is Professor J.
SPEAKER_4: Richard Gott from Princeton University.
SPEAKER_4: He's Professor of Astrophysics there.
SPEAKER_4: He's written a book called Time Travel in Einstein's Universe.
SPEAKER_4: So we're all about that today.
SPEAKER_4: But first, let me ask you, you're from Princeton.
SPEAKER_4: I noticed you have a distinct accent from south of the Mason-Dixon line.
SPEAKER_4: Where is that from?
SPEAKER_2: I'm from Louisville, Kentucky, originally.
SPEAKER_4: Louisville.
SPEAKER_2: Louisville.
SPEAKER_2: There are many different ways to pronounce this.
SPEAKER_4: Louisville, Kentucky.
SPEAKER_4: How many other Kentucky astrophysicists are there?
SPEAKER_2: Well, Hubbell actually spent a little time in Louisville teaching high school early in his career.
SPEAKER_4: But he surely did not speak with a Kentucky accent.
SPEAKER_2: No, he kind of had an English accent after he spent some time in England.
SPEAKER_4: Sounds like he enjoyed faking that English accent.
SPEAKER_2: My accent hasn't changed that much since I've been in New Jersey.
SPEAKER_4: People might remember that Princeton is where Einstein spent out his later years.
SPEAKER_4: He came to America and hung out at Princeton.
SPEAKER_4: He still revered there, of course, right?
SPEAKER_4: Is that right?
SPEAKER_2: Oh, sure.
SPEAKER_2: At the 50th, we had the 50th anniversary of his 1905 theory of special relativity, and they actually put up a bust of Einstein off the main street there in Princeton.
SPEAKER_4: Okay, did people genuflect in front of it?
SPEAKER_4: Or did they?
SPEAKER_2: Princeton people loved Einstein.
SPEAKER_2: He would eat at the restaurants, and he would go into the flower shop and order like 17 roses, you know.
SPEAKER_4: Just for all his girlfriends?
SPEAKER_4: I heard he was the ladies' man.
SPEAKER_2: Well, he did order flowers at the flower shop, and the trouble was that no one would cash his checks, because a check signed by Einstein was worth more than the amount.
SPEAKER_2: So finally, he said, listen, could I clear up my checkbook?
SPEAKER_2: I'll give you as many autographs as you want, and you can just cash the checks.
SPEAKER_4: So the Einstein t-shirts, so there soon will be like Richard Gott t-shirts in Princeton, do you think?
SPEAKER_2: Well, I haven't seen one yet.
SPEAKER_4: So Rich, tell me, not only can you time travel by moving fast, out of Einstein's theories, general theory of relativity, gravity, extreme gravity can also affect your time travel.
SPEAKER_4: That's true as well.
SPEAKER_4: So any examples of that that you can cite?
SPEAKER_2: Well, if you go deep in a gravitational well...
SPEAKER_4: Just closer to a source of gravity.
SPEAKER_2: Closer to a source of gravity.
SPEAKER_2: For example, if you were on the planet Mercury, your clock would tick slightly slower there, both because Mercury is orbiting around the Sun rapidly, more rapidly than the Earth, but also because Mercury is down, hunkered down in the gravitational well of the Sun.
SPEAKER_2: So when photons come out from Mercury to us, they lose energy because they got to climb out of this gravitational well and that lengthens their wavelength and it means that the waves come in from wavelength to wavelength more slowly.
SPEAKER_2: So you see everything going in slow motion, so I'm talking to you very slowly from Mercury.
SPEAKER_4: If you were a Mercury resident and didn't vaporize in the heat of the Sun, that's what you would sound like.
SPEAKER_2: A little slowly.
SPEAKER_2: So if we sent an astronaut to live on Mercury for 30 years and then brought them back to Earth, they'd come back about 22 seconds younger than they should have been, or they'd travel 22 seconds into the future.
SPEAKER_4: So if, ignoring the speed version of that time travel, just the being in a deep gravity well, he can just sit there and age more slowly.
SPEAKER_2: That's true.
SPEAKER_4: So is any movie that sort of captures that aspect of it?
SPEAKER_2: Well, yes.
SPEAKER_2: There was a movie, The Time Machine, in 2001.
SPEAKER_4: Wells, The Time Machine.
SPEAKER_2: It's the same story of HG.
SPEAKER_2: Wells, basically.
SPEAKER_2: With a few additions.
SPEAKER_2: Basically the same story.
SPEAKER_2: Uh-huh.
SPEAKER_2: And he sits in this sort of spherical time machine, and he just notices that the world outside is going by rather quickly, and namely he's going slowly through time.
SPEAKER_2: He's aging more slowly.
SPEAKER_2: Now you could really build one like this, about that size that's depicted in the movie.
SPEAKER_2: If you put yourself inside a shell of mass that was about the mass of Jupiter.
SPEAKER_2: Now you would age four times more slowly than the people outside, so it's a good way to hurry to the next election, you know?
SPEAKER_2: You could get there quicker.
SPEAKER_4: You could get there one year instead of four years.
SPEAKER_2: Yeah, it would only take you one year to get there.
SPEAKER_4: So that's just Jupiter, but imagine a black hole would accelerate this, right?
SPEAKER_2: Well, yes, if you wanted to go millions of years into the future, as the movie portrays him going 800,000 years into the future.
SPEAKER_2: For that one, you need about 100 million solar masses, and it's going to be about the size, this shell of mass is going to be about the size of the Earth's orbit.
SPEAKER_2: And you're going to sit in there and age more slowly.
SPEAKER_2: And the other way to do is...
SPEAKER_4: And by the way, it's one thing to say you age more slowly, but what's actually going on is you think you're being normal, and everything around you is moving faster.
SPEAKER_2: That's right.
SPEAKER_2: And one of the...
SPEAKER_4: I think we have Michael on the line from Los Angeles.
SPEAKER_6: Yeah.
SPEAKER_4: Is that you?
SPEAKER_4: How are you doing?
SPEAKER_4: Welcome to StarTalk.
SPEAKER_6: Pretty good.
SPEAKER_6: You sort of answered my question.
SPEAKER_6: I guess what I, as you approached heavier gravitational sources, my question is, is it the physical ramifications that if you were on Mercury, your body is going to have to expend more energy, i.e.
SPEAKER_6: that parts of your body is going to break down faster, muscles and all that.
SPEAKER_6: So I would think physically, well, obviously, Mercury or, but I'm even, I guess I'm saying how do you counteract that physical force that would be detrimental to your body?
SPEAKER_4: Plus there's a little bit about the fact that the sun would vaporize you on the surface.
SPEAKER_2: Right, right.
SPEAKER_6: But I guess it's, is it the gravity that actually does this or is there, you know, like Einstein said, the fabric of that universe and stuff is being stretched.
SPEAKER_4: Michael, that's an excellent question.
SPEAKER_4: So Rich, what he's asking is, thank you, Michael.
SPEAKER_4: What he's asking is, is it something physiological about the heavier gravity that your body is reacting to or is there something deeper than that?
SPEAKER_2: Well, you think everything's going normally.
SPEAKER_2: We're going to put you in a good space cabin, you know, to live.
SPEAKER_2: We're going to protect you from the heat of the sun and that.
SPEAKER_2: But the thing that's going to happen to you is that as you live out your life, if you send signals out to me, those are going to lose energy as they come out.
SPEAKER_2: So their wavelength is going to be longer.
SPEAKER_2: So I'm going to look at you and see everything happening in slow motion.
SPEAKER_2: And you're going to look back at the Earth and see everything happening a bit fast.
SPEAKER_2: So the time travel movie that we were talking about, where he was seeing thousands of years go by on the surface of the Earth and he was aging only very slowly, those visible photons coming in showing him the visible light coming in would be shifted way into the ultraviolet and into the X-rays.
SPEAKER_2: So he'd be fried by looking at the vicinity of the Earth.
SPEAKER_4: They missed that part of the movie.
SPEAKER_2: They missed that part and also the part that his spaceship would have been so massive that it would have torn the Earth apart.
SPEAKER_2: So I do not expect to find people building time machines in their garage.
SPEAKER_4: Now, have you ever consulted on a time machine movie?
SPEAKER_4: I mean, now that you're author of Time Travel in Einstein's Universe.
SPEAKER_2: I've consulted on other movies and the thing is they usually ask for your advice and then they don't follow it because the plot needs to be spicier.
SPEAKER_2: That's the trouble.
SPEAKER_4: So tell me about how black holes would play into this.
SPEAKER_4: If I orbit a black hole real close, I can be moving fast and I'm in a deep gravitational well.
SPEAKER_4: So how different is the time there?
SPEAKER_2: Well, you can slow down your clock by just hanging out in the vicinity of the black hole.
SPEAKER_2: Don't go in.
SPEAKER_2: Don't go past that point of no return.
SPEAKER_2: But you hang out outside and you can get quite a bit toward the future doing that.
SPEAKER_2: So that's a good way to go toward the future.
SPEAKER_2: You want to get a big black hole so that the tidal forces aren't too bad.
SPEAKER_4: Otherwise, you'd be ripped apart.
SPEAKER_2: You don't want to be ripped apart.
SPEAKER_2: But we got three billion solar mass black holes for the tidal forces at the edge are quite fine.
SPEAKER_4: We've got those in the centers of galaxies.
SPEAKER_2: Yes.
SPEAKER_4: So we can send you there and report back.
SPEAKER_2: We have to go there and then hang around.
SPEAKER_4: You're listening to StarTalk Radio.
SPEAKER_4: I'm your host, Neil deGrasse Tyson, with my guest co-host this week, Professor J.
SPEAKER_4: Richard Gott III, if I remember correctly, professor of astrophysics at Princeton University.
SPEAKER_4: He's telling us about time travel, the astrophysics of it.
SPEAKER_4: You can call in if you have a question about black holes, the universe, the Big Bang, ways of traveling forward or backwards in time.
SPEAKER_4: We're at 1-877-5-STARTALK.
SPEAKER_4: You can also hear us stream on startalkradio.net, or you can tweet us.
SPEAKER_4: If you have a tweet, send it to StarTalk Radio.
SPEAKER_4: We're there.
SPEAKER_4: So Rich, there's other movies where they go into the future.
SPEAKER_2: See, if you're coming from a black hole, it'd be stretched out.
SPEAKER_4: Tweet.
SPEAKER_2: We get one like that, they're coming from a black hole.
SPEAKER_2: Very slow tweet.
SPEAKER_4: Anyone from the future or from the...
SPEAKER_4: yeah, tweet us, and we'll know by the frequency of your sound.
SPEAKER_4: Tell me about one of everyone's early favorite time travel movies, which was Planet of the Apes.
SPEAKER_4: Is that just a simple time travel into the future movie?
SPEAKER_2: That's the simplest kind.
SPEAKER_2: The astronaut Charlton Heston, he goes in a spaceship, you know, it gets going too fast, near the speed of light.
SPEAKER_2: He doesn't really know that.
SPEAKER_2: He comes back, he lands on the Earth.
SPEAKER_4: Oh, he doesn't know it's Earth.
SPEAKER_2: He doesn't know it's Earth.
SPEAKER_2: He thinks it's just landed on another planet because there's these apes running everything.
SPEAKER_2: And then, of course, I had to give away the end of the movie, but he...
SPEAKER_4: But what would have cued him in is that the apes are speaking English.
SPEAKER_2: That would be a hint.
SPEAKER_2: You'd think he would pick up on that, you know?
SPEAKER_2: Why are the apes speaking English?
SPEAKER_2: How do they know English?
SPEAKER_2: But he finally, at the end of the movie, he encounters the ruins of the Statue of Liberty, and then he says, oh my.
SPEAKER_4: So this is simple forward time travel using Einstein.
SPEAKER_2: Easy to do.
SPEAKER_2: We know you can do this.
SPEAKER_2: It's just a matter of money.
SPEAKER_2: You got to increase the NASA budget by a lot.
SPEAKER_4: So you get to moving really fast.
SPEAKER_4: We've got another call.
SPEAKER_4: John, is that you in LA?
SPEAKER_7: Yes, it is.
SPEAKER_4: How are you doing?
SPEAKER_4: What questions do you have for us today?
SPEAKER_7: Oh, I have a question which has troubled me for a long time, and I'm hoping that somebody can answer it.
SPEAKER_4: Bring your troubles onto us.
SPEAKER_7: I understand the Universe to have, at the time of the Big Bang, been a very, very tiny point.
SPEAKER_7: And then we had the Big Bang, and since the Big Bang, according to modern science, I believe 13.7 billion years have passed.
SPEAKER_4: You got it.
SPEAKER_7: And with modern telescopes, we are able to see probably more than 90% back in time to galaxies that are on the very edge, practically at the edge of the Universe.
SPEAKER_4: You got it.
SPEAKER_7: Those galaxies are perhaps 13 billion light years away.
SPEAKER_4: You can teach Astro 101 at the rate you're going, sir.
SPEAKER_4: It's not clear you will have any question at all at the end of this.
SPEAKER_7: Here is my question, sir.
SPEAKER_7: Starting from a point in 13, how did those galaxies get 13 billion light years away in approximately 13 billion light years away so quickly?
SPEAKER_4: Okay, so Rich, so he's asking...
SPEAKER_2: This is a very good and very deep question.
SPEAKER_4: And is there time travel going on with these galaxies?
SPEAKER_2: Well, what's...
SPEAKER_4: Yeah, thank you, John, for that question.
SPEAKER_4: Rich, what do you have?
SPEAKER_2: Well, what's happening here is that in the very early universe, the galaxies are moving apart from each other because the space between the galaxies is stretching like a big sheet of rubber.
SPEAKER_2: And so we see the galaxies, with time, getting further and further away from us because this sheet of rubber is stretching.
SPEAKER_2: So at the beginning of the universe, the universe is stretching so fast that the light can't travel between the two points.
SPEAKER_2: So it's...
SPEAKER_2: the space is stretching between those two galaxies faster than light could cross the distance between them.
SPEAKER_2: Einstein's theory says you can't go faster than the speed of light.
SPEAKER_2: Someone can't pass you faster than the speed of light.
SPEAKER_2: But it never says that the space between you and someone else can't stretch faster than the speed of light.
SPEAKER_2: So that's how they get out there for us to see.
SPEAKER_4: So you're talking about the fabric of the universe expanding at any arbitrary speed according to the tenets of the Big Bang.
SPEAKER_2: Because little pieces of it are just stretching a little.
SPEAKER_2: But you put a lot of them together, and that makes the distance between the two stretch faster than the speed of light.
SPEAKER_4: Well, I'm ready to start giving away your book, actually.
SPEAKER_4: In fact, I wanted to give it to John, but he hung up before we could get his address.
SPEAKER_4: In fact, if he calls back, I think we can give his address.
SPEAKER_4: I think we'll give him a book.
SPEAKER_4: Because that was a good question, you admit.
SPEAKER_2: That was a great question.
SPEAKER_4: John, if you're out there, let us know.
SPEAKER_4: We're going to take a break, and we're going to come back to StarTalk Radio.
SPEAKER_4: If you want to call us, you have a question, a comment about what your favorite time travel movie is, a question for my co-host for this episode, J.
SPEAKER_4: Richard Gott, professor of astrophysics, give us a call at 1-877-5-STAR-TALK.
SPEAKER_4: We're streamed on startalkradio.net, and you can tweet us at StarTalk Radio.
SPEAKER_4: See you in a bit.
SPEAKER_1: The future of space and the secrets of our planet revealed.
SPEAKER_1: This is StarTalk.
SPEAKER_4: Welcome back to StarTalk.
SPEAKER_4: I'm your host, Neil deGrasse Tyson, astrophysicist, with a friend and colleague of mine, Professor J.
SPEAKER_4: Richard Gott, Professor of Astrophysics at Princeton University, as guest co-host, substituting for Lynn Coplitz, who's actually doing stand-up comedy tonight at the Pittsburgh Improv.
SPEAKER_4: Check her out if you're in town.
SPEAKER_4: So Rich, I've got a couple of questions for you.
SPEAKER_4: Are your students getting smarter or dumber lately?
SPEAKER_4: We hear all these complaints about America getting dumb.
SPEAKER_4: When you teach astrophysics, how do you feel about them?
SPEAKER_4: Over the years?
SPEAKER_2: I think people today are just as smart as they were before.
SPEAKER_2: About the same is the answer.
SPEAKER_4: Has there been like an Einstein in your class that you could identify as somebody?
SPEAKER_2: Well, they come along more rarely than that.
SPEAKER_4: Okay, I was just curious.
SPEAKER_4: I used to be in the professor role, but now I don't see a new crop of students every year.
SPEAKER_4: But if the trend line is about stable, that's a good sign, I think.
SPEAKER_4: So let me ask you.
SPEAKER_4: We were talking about time travel, and not to be too topical, but we lost Michael Jackson recently.
SPEAKER_4: He and I are the same age, by the way.
SPEAKER_4: I don't know if you know, but he and I were once confused for each other back when I was 13 years old.
SPEAKER_2: Well, that's interesting.
SPEAKER_4: I was left less sort of beefed up back then before I started doing athletics.
SPEAKER_4: So I was kind of a skinny kid, and we had the same sort of skin tone and had the same afro of the day.
SPEAKER_4: And three girls, I was in a, in a, in a, in Gimbels, the department store.
SPEAKER_4: And these three girls were like drawing straws as he would walk up to me to see if I was Michael Jackson.
SPEAKER_4: And one of them came up, and I had to disappoint them.
SPEAKER_2: Well, you must have looked very self-confident.
SPEAKER_4: Certainly before all the surgery.
SPEAKER_4: I'm wondering if it's one thing to send your whole body forward and back in time, but could, like, Michael Jackson get his old nose back or something?
SPEAKER_4: Is there a way to do something like that?
SPEAKER_2: Well, no, when you, the time traveler, even though you go visit the past, you're always going forward into the future.
SPEAKER_4: Into your own personal future.
SPEAKER_2: Into your own personal future.
SPEAKER_2: So you keep aging.
SPEAKER_2: It's like Magellan, in general relativity, we have curved spacetime.
SPEAKER_2: So it's like Magellan's crew left Europe.
SPEAKER_2: They went west, west, west, around the world, and they came back to Europe again.
SPEAKER_2: So the time traveler goes forward, always toward the future, as far as he's concerned, but circles back through twisted, curved spacetime.
SPEAKER_4: Like the surface of the Earth was.
SPEAKER_2: Just like the surface of the Earth was, to an event in his own past.
SPEAKER_2: So you can visit the past, but you'll be older when you get back there.
SPEAKER_4: You've flushed your nose and everything else that Michael Jackson might have wanted back.
SPEAKER_2: You don't age in reverse when you go back in time.
SPEAKER_4: Now, tell me about wormholes.
SPEAKER_4: We've always heard about, you know, in Contact, Jodie Foster looks like she travels through a wormhole.
SPEAKER_4: There's the warp space in Star Trek IV.
SPEAKER_4: Let's go back in time.
SPEAKER_4: How's that possible?
SPEAKER_2: Well, in general relativity, this is Einstein's theory of curved spacetime to explain gravity.
SPEAKER_2: And we believe this because it's been checked by a light bending around the sun.
SPEAKER_2: We've done experiments that have checked this.
SPEAKER_4: That means we know it.
SPEAKER_4: We don't have to just believe it.
SPEAKER_2: That's right.
SPEAKER_2: It's past the experimental test.
SPEAKER_2: So general relativity, you have curved spacetime.
SPEAKER_2: So there was a famous poem.
SPEAKER_2: There was a young lady named Bright.
SPEAKER_2: She traveled far faster than light.
SPEAKER_2: She left one day in a relative way and returned home the previous night.
SPEAKER_4: Okay, there she goes.
SPEAKER_2: So we always knew that if you could go faster than the speed of light somehow, you could go back in time theoretically.
SPEAKER_2: But Einstein showed that you couldn't build a rocket that would go, that would pass someone at faster than the speed of light.
SPEAKER_2: That's because if you were in the rocket and you shot a laser beam toward the front of the rocket, it would never catch up to the front of the rocket.
SPEAKER_4: It would go backwards in your rocket, right?
SPEAKER_2: That's right.
SPEAKER_2: It would hit the back end of the rocket.
SPEAKER_2: And so you'd know you were moving.
SPEAKER_2: And that's not allowed by that first postulate.
SPEAKER_2: So you can't build a rocket that goes faster than the speed of light.
SPEAKER_2: But if you can take a shortcut by jumping through a wormhole and get to a distant place in space effectively faster than light would have, then you beat a light beam there.
SPEAKER_4: Is that what they did in Star Trek IV?
SPEAKER_4: Star Trek IV, just to remind you, you know, there's a current Star Trek movie out.
SPEAKER_4: I saw it and I loved it.
SPEAKER_4: Star Trek IV was like the Save the Whales movie.
SPEAKER_4: Remember that one?
SPEAKER_4: They go back to 1984 because there's a whale sound in their future time and the aliens picked it up and they're waiting for the whale to reply and there are no whales left because we hunted them to extinction and so there's a problem.
SPEAKER_2: There's a problem.
SPEAKER_2: And so what they did, of course, was they went back in time.
SPEAKER_2: They got a whale.
SPEAKER_2: They brought it to the 23rd century.
SPEAKER_4: Much to the whale's surprise.
SPEAKER_2: And the whale answers the extraterrestrial.
SPEAKER_4: Well, you gotta bring two whales.
SPEAKER_4: You need two whales.
SPEAKER_2: Well, they brought two.
SPEAKER_2: But anyway, it answered the extraterrestrial with the whale song.
SPEAKER_2: And so everything was fine.
SPEAKER_2: My theory of that movie was always that the trouble started when Carl Sagan sent on one of the Voyager spacecraft a record with everyone on Earth saying hello, hola, in all the different languages.
SPEAKER_2: And they sent the whale song.
SPEAKER_2: And this was a big mistake because we don't know what the whales are saying.
SPEAKER_4: We know what each of us cells are saying.
SPEAKER_4: Even in our multiple languages.
SPEAKER_4: But what are you worried about?
SPEAKER_4: It's just a whale song.
SPEAKER_2: Well, they could be saying, help, the humans are running an extinction, come and save us.
SPEAKER_2: Which I think was the basic plot of this movie.
SPEAKER_2: So they went to the past, they got the whales, and everything was fine.
SPEAKER_2: This is totally self-consistent.
SPEAKER_4: Okay, alright.
SPEAKER_4: And so that movie was okay.
SPEAKER_4: They did it right.
SPEAKER_4: But the whale was a little surprised to be yanked out of the present.
SPEAKER_2: And you have a whale that disappears from San Francisco one day, and they say, where'd the whale go?
SPEAKER_2: And that's about it.
SPEAKER_4: That's okay.
SPEAKER_4: So could it be that there are people who have just disappeared that are sort of open cases in police log that you think someone has zapped them into the future?
SPEAKER_4: Like Jimmy Hoffa.
SPEAKER_4: No one can find Jimmy Hoffa.
SPEAKER_4: Is he in the future?
SPEAKER_2: Maybe they should be looking for him in the 23rd century.
SPEAKER_2: In New Jersey.
SPEAKER_2: In the 23rd century.
SPEAKER_2: Be a good place to hide.
SPEAKER_4: So Wormholes, do they exist?
SPEAKER_4: Can we make it?
SPEAKER_4: I gotta give out the phone number here, because I can't be the only one wondering how to ask questions about this.
SPEAKER_4: You're listening to StarTalk.
SPEAKER_4: I'm your host, Neil deGrasse Tyson.
SPEAKER_4: We've got a question for my co-host for this session, J.
SPEAKER_4: Richard Gott, professor of astrophysics and expert on everything cosmic, but especially on time travel, author of Time Travel in Einstein's Universe.
SPEAKER_4: Give us a call at 1-877-5-StarTalk.
SPEAKER_4: So, tell me, is...
SPEAKER_4: is...
SPEAKER_4: what...
SPEAKER_4: in time travel, wormholes, can we make a wormhole?
SPEAKER_2: Well, the wormhole is made.
SPEAKER_2: You have to have something to prop it open or else it will collapse like a black hole and kill you.
SPEAKER_2: So, we need some negative energy density stuff, some stuff that weighs less than zero, to hold it open.
SPEAKER_4: Because gravity would hold it closed.
SPEAKER_4: So, you need something to fight gravity.
SPEAKER_2: You need something to fight gravity and hold it open.
SPEAKER_2: And in fact, the empty space between two parallel metal plates is stuff like that.
SPEAKER_2: The vacuum between those two plates is stuff that weighs less than zero.
SPEAKER_2: It's called the Casimir effect.
SPEAKER_4: So, we know this stuff exists.
SPEAKER_2: Yeah, that's what Kip Thorne's using to prop open the wormhole and allow you to go through.
SPEAKER_4: Oh, so, okay.
SPEAKER_4: But you got to keep it open, otherwise it collapses on you and you're dead.
SPEAKER_4: It's dangerous stuff.
SPEAKER_2: And one mouth of it is like here near the Earth and the other one might be four light years away.
SPEAKER_2: So you jump in the wormhole, you travel about ten feet and you pop out near Alpha Centauri.
SPEAKER_4: That works.
SPEAKER_2: That works.
SPEAKER_2: And you then have beaten a light beam there.
SPEAKER_2: And so if you move the wormhole mouths around appropriately, you can build a time machine.
SPEAKER_4: We've got a caller.
SPEAKER_4: Another one.
SPEAKER_4: John, Tucson.
SPEAKER_4: Is that Tucson, Arizona, John?
SPEAKER_7: No, Tuscan, California.
SPEAKER_4: Oh, excuse me, Tuscan.
SPEAKER_4: Tuscan, California.
SPEAKER_4: Excuse me.
SPEAKER_4: So, John, you've got a question for Professor Gott.
SPEAKER_7: Yeah, I was wondering about the theory of relativity and there not being any absolute motion.
SPEAKER_7: Wouldn't rotary motion be a form of absolute motion?
SPEAKER_7: Because if you were like blindfolded in space and had like a jet pack or something and started spinning in a circle, wouldn't the effect, your arms going out, wouldn't that prove to you that you're moving without any outside influence?
SPEAKER_4: Yeah, excellent question, John.
SPEAKER_2: That's true.
SPEAKER_2: You can tell whether or not you're rotating or not because you get dizzy.
SPEAKER_4: And throw up.
SPEAKER_2: And we have the thing in the inner ear to tell you that.
SPEAKER_2: Einstein's postulate of special relativity assume uniform motion in a straight direction without turning.
SPEAKER_2: So when you write the details of his postulate, it's stated that way.
SPEAKER_4: So therefore, circular motion, rotational motion, if you're in a spin dryer, you know.
SPEAKER_4: And that doesn't count in this analogy.
SPEAKER_2: You could use a spin dryer if you hopped in one of those and you're spinning around rapidly.
SPEAKER_2: A little time travel to the future.
SPEAKER_2: Not much.
SPEAKER_4: Is a spin dryer the same effect as being...
SPEAKER_4: So in other words, if you're in a spin dryer spinning really fast, does that simulate being in a strong gravitational field?
SPEAKER_2: Yes, it does.
SPEAKER_2: In fact, Einstein showed that those two things are really the same.
SPEAKER_2: That acceleration you get on one of those whirligigs at the amusement park where you're pressed back hard against the back.
SPEAKER_2: He said, I think that's all gravity is.
SPEAKER_2: It's that kind of thing.
SPEAKER_2: It's the same physical thing going on.
SPEAKER_2: That was his genius in general relativity.
SPEAKER_4: Okay, so I don't know, John, if you're still on the line, but that's, so we have the correspondence between rotary motion and gravity.
SPEAKER_4: And you can still age more slowly either on a black hole or in a spin dryer.
SPEAKER_4: That's what you're telling us.
SPEAKER_2: Yes, and particles that go around in our accelerators, they're going around in a circle and they age much more slowly because they're going near the speed of light.
SPEAKER_4: So, Rich, we've got a tweet.
SPEAKER_4: Thank you.
SPEAKER_4: Okay, thanks, John, for calling in.
SPEAKER_4: And if you hang on, we'll get you a book to you.
SPEAKER_4: If you can give us your address before you hang up.
SPEAKER_4: If you hung up, call back in.
SPEAKER_4: We'll give you one of Rich Gott's book, Time Travel in Einstein's Universe.
SPEAKER_4: We have a tweet, Rich.
SPEAKER_4: Let me read it to you.
SPEAKER_4: If you were to go back in time and change something involving yourself, would you remember telling you something and not remember you being back in time?
SPEAKER_4: Let me see what that means.
SPEAKER_4: We're trying to approach a paradox here.
SPEAKER_4: Are there paradoxes?
SPEAKER_4: For example, Bill and Ted's Excellent Adventure.
SPEAKER_4: There's a movie.
SPEAKER_4: You gotta love Bill and Ted.
SPEAKER_4: I don't know who remembered.
SPEAKER_4: Netflix did just last weekend.
SPEAKER_4: The whole family saw it.
SPEAKER_4: They go back in time, collect these old historic dudes, bring them into the present, but they also meet themselves at a point back in time.
SPEAKER_4: It sounds like that's a paradox.
SPEAKER_4: Suppose they prevented their parents from meeting, for example, then they wouldn't be born, and then they couldn't go back in time to prevent them parents from meeting.
SPEAKER_4: How does that work?
SPEAKER_2: Well, this is basically your grandmother paradox.
SPEAKER_2: Suppose you kill your grandmother as a young girl.
SPEAKER_4: Yes, if I go back in time.
SPEAKER_2: Then you won't be born.
SPEAKER_4: Born and alive to go back and kill my grandmother to prevent me from being born.
SPEAKER_2: That's right.
SPEAKER_2: But if you kill your grandmother, and then you're not born, then your whole world line, your whole path through space and time vanishes, and you're not there to kill your grandmother.
SPEAKER_2: She says, it's all right after all.
SPEAKER_2: So that didn't really solve the paradox.
SPEAKER_4: It's still a paradox.
SPEAKER_2: So there are two solutions to the paradox.
SPEAKER_2: One is that the world has to be self-consistent.
SPEAKER_2: Time travelers don't change the past at all.
SPEAKER_2: They were always part of it, like the person tweeting in said.
SPEAKER_2: If you met yourself in the past, you have a recollection of that.
SPEAKER_2: You've written that down in your diary.
SPEAKER_2: And so you're not, the time traveler doesn't change the world at all.
SPEAKER_4: So what you're telling me, Rich, is I know my past.
SPEAKER_2: That's right.
SPEAKER_4: I know, therefore, that I will not build a time machine and meet myself in my current past, because that hasn't happened in my past.
SPEAKER_2: That's right.
SPEAKER_4: So therefore, no one alive today, unless they can tell you they've met themselves in the past, has met themselves in the past.
SPEAKER_4: Or will in the future meet themselves in the past of the present?
SPEAKER_2: That's right.
SPEAKER_2: So it doesn't mean time travelers can't influence.
SPEAKER_4: Well, let me give out our phone number again.
SPEAKER_4: Plus, we've got to take a quick break.
SPEAKER_4: If you've got a question about time travel, any of these movies we've been talking about, Relativity, give us a call on Star Talk Radio, 1-877-5-STARTALK.
SPEAKER_4: Tweet us at Star Talk Radio.
SPEAKER_4: We'll be back in a bit.
SPEAKER_4: Welcome back to StarTalk.
SPEAKER_4: I'm your host, Neil deGrasse Tyson, with a special guest host sitting in for Lynn Coplitz.
SPEAKER_4: This week, it's Professor Richard Gott, the third Professor of Astrophysics at Princeton University, a colleague of mine from the years I spent on the faculty there.
SPEAKER_4: And he's not only an expert on all things cosmic, but a particular expert on time travel, having written a book called Time Travel in Einstein's Universe.
SPEAKER_4: And we'd like to give the book away to callers that we take online here.
SPEAKER_4: So Rich, tell me something.
SPEAKER_4: This, if you go back in time, this paradox problem, if you can't kill your, you prevent your parents from meeting, if I know since my parents did meet and I was born, I can't go back in time and prevent that.
SPEAKER_2: No.
SPEAKER_4: So therefore, if I'm gonna go back in time, it has to be in a time machine that is yet to be invented and I go back in time of the past of that but not the past of my current present.
SPEAKER_2: Yeah, one of the things we've discovered is, that's interesting, is that you can't go back before the time machine was invented.
SPEAKER_2: So if we invent a time machine in the year 3000 by twisting space and time up there, we can't use it to come back to the present.
SPEAKER_2: We can use it to go from 3002 back to 3001, but not before 3000 when the time machine was invented.
SPEAKER_4: So if a time machine has not yet been invented, nobody's going back to visit Julius Caesar.
SPEAKER_2: That's right, that's why you don't see, it's sufficient to explain why you don't see time travelers at the Kennedy assassination or other historical events.
SPEAKER_4: Like the Titanic.
SPEAKER_2: Well yes, if you had a time machine, and you built it before the Titanic had sailed, then you could use it, and in fact, one wag suggested that the real reason that Titanic sank was all the time traveler stowaways on board that added to the weight of the ship, so it sank because of the time travelers there to witness the sinking.
SPEAKER_4: This is crazy.
SPEAKER_4: We've got a call, Ron from LA, is that you on the line?
SPEAKER_8: Yes, hello.
SPEAKER_4: Hi, welcome to StarTalk.
SPEAKER_8: Oh, I'm delighted, it's a wonderful conversation.
SPEAKER_8: My question for your guest, for both of you, is this.
SPEAKER_8: We're talking a lot about time and time travel.
SPEAKER_8: I think many of us human beings experience time as having a past where there are no possibilities, a vivid thing, which we call the present, and then a future where there are no facts, okay?
SPEAKER_8: In other words, they're different, past, present, future, as I experience it.
SPEAKER_8: How do you reconcile that kind of time with the time that you're talking about with people aging at different rates and so forth?
SPEAKER_4: That's an excellent question.
SPEAKER_4: Certainly worthy of a book.
SPEAKER_8: I'd love to read it.
SPEAKER_4: Wrong, okay.
SPEAKER_4: So Rich, yeah, so we've got this past, present, and future which we experience differently physiologically or neurologically, yet you're talking about this whole continuum of time.
SPEAKER_4: How did they square with each other?
SPEAKER_2: Well, Einstein showed that time, there was a minus sign associated with the dimension of time.
SPEAKER_4: Because we have height, width, depth, and time.
SPEAKER_4: And it had a minus sign.
SPEAKER_2: That just had a minus sign.
SPEAKER_4: In its equations.
SPEAKER_2: That's all it is, mathematically.
SPEAKER_2: And yet the human perception of it is that like time is like a movie.
SPEAKER_2: Now, the past we know about because light beams go forward in time.
SPEAKER_2: There is an arrow of time that we think is connected with somehow the initial conditions of the universe.
SPEAKER_2: And light beams go forward in time, not backwards.
SPEAKER_2: So events, something causes something over here and then the effects occur later.
SPEAKER_2: So that's why we know about the past and not about the future.
SPEAKER_4: Whereas we can know where we came from and where we're going because you can see the block that you walked and the block you're about to walk.
SPEAKER_4: But we don't have that access to the time dimension of all of this.
SPEAKER_2: We don't have access to the future yet, but the future in Einstein's view, it was one four-dimensional thing.
SPEAKER_2: So there was one past and one future.
SPEAKER_2: We do have ideas now about many worlds theory of quantum mechanics.
SPEAKER_2: There could be many parallel worlds.
SPEAKER_2: But Einstein's view was that there was just one four-dimensional thing, space-time, and it played out like a movie.
SPEAKER_4: Okay, so the two very popular movies, Back to the Future and Terminator, they're both going into the past in those movies.
SPEAKER_4: How did they square in terms of the astrophysics?
SPEAKER_2: Well, in the Back to the Future movie, at one point when he stops his parents from meeting, his hand disappears.
SPEAKER_2: Well, hands just don't disappear like that.
SPEAKER_2: And besides, his whole path through space-time would have disappeared.
SPEAKER_2: So they didn't quite solve that problem, but he did get his parents successfully back together.
SPEAKER_4: One of my favorite lines is there's the estate where his homes were built.
SPEAKER_4: I think at the beginning it was Twin Pines estate, but one of the pine trees got knocked over when the DeLorean landed in that undeveloped land, knocked over one of the pine trees, so later on it becomes Lone Pine estate.
SPEAKER_2: So they're working in the many worlds picture.
SPEAKER_4: There's the Lone Pine shopping center, that's what it was.
SPEAKER_2: Yeah, so they're working on the idea that there are many parallel worlds, and when he goes back in time, he causes a branch off on the parallel world.
SPEAKER_4: A separate universe that's otherwise identical except for these details.
SPEAKER_2: Right.
SPEAKER_4: Interesting.
SPEAKER_4: And Terminator, so they, that one, we love Terminator.
SPEAKER_4: You gotta love Terminator.
SPEAKER_2: Well, yes, the Terminator came back to try to kill Mrs.
SPEAKER_2: Connor, so she wouldn't give birth to John Connor, who was the leader of the human revolution to fight back against the machines.
SPEAKER_4: Who turns out to be the father of the guy who was sent back to, that got really creepy.
SPEAKER_2: That guy who went back turned out to be John Connor's father that was very convenient.
SPEAKER_4: Rich, we're almost out of time as the license plate of the professor in Back to the Future.
SPEAKER_4: What was his name again?
SPEAKER_2: I forget his name.
SPEAKER_4: How could you forget his name?
SPEAKER_4: Yeah, how could we forget the guy's name?
SPEAKER_4: I forget his name.
SPEAKER_4: Anyhow, so let me tell you, this has been a great show, Rich.
SPEAKER_4: I mean, going back in time, it's nice to know that it may be possible one day.
SPEAKER_4: It's not possible yet.
SPEAKER_4: And we're not gonna be able to visit major historical events in anybody's past right now.
SPEAKER_4: Unless of course, the government has a secret time machine.
SPEAKER_4: But I've spent some time in the government.
SPEAKER_4: They're not nearly as competent as anyone ever holds them up for being.
SPEAKER_4: Any final reflectance on this?
SPEAKER_2: Well, it's been a nice time here with you, Neil.
SPEAKER_4: Well, thank you, Rich.
SPEAKER_4: You've been listening to StarTalk, a radio program funded by the National Science Foundation.
SPEAKER_4: You can track us on startalkradio.net, where you can hear past shows and get the schedule for upcoming shows.
SPEAKER_4: There you have it.
SPEAKER_4: Check us out next week when Link Hoplix returns.
SPEAKER_4: A reminder, tonight she's performing live at the Pittsburgh Improv, Pittsburgh, Pennsylvania.
SPEAKER_4: We'll see you next week.
SPEAKER_4: Thanks for coming.